MILD WINTERS. 41 



be followed in covering all half-hardy trees or 

 shrubs. It is not necessary to shield them from 

 the cold, which one would judge was the object 

 from the immense bundles of straw with which many 

 such plants are surrounded, but to shade them so 

 as to prevent those sudden changes, from heat to 

 extreme cold, which destroy the tissue. 



Mild winters are those most destructive to vege- 

 tation, as they are generally characterized by great 

 reverses of temperature. A writer says : ^ " We 

 speak of one year as warmer or colder than another; 

 but it is a wonderful example of unchanging law, 

 that they seldom differ materially in the mean tem- 

 perature of the year. That of London is fifty de- 

 grees, four minutes ; and, however hot or cold the 

 seasons were, it did not cause the average of the 

 year to vary more than one-half a degree ; and this 

 was probably owing to the imperfection of our im- 

 plements." 



A warm summer is very beneficial to vegetation, 

 because, as we have already seen, a constant high 

 temperature at that time stimulates their vital en- 

 ergies. But as the mean of every year is nearly the 

 same, in order to have a warm summer the preced- 

 ing winter must have been cold. Sometimes, how- 

 ever, the heat following a cold winter seems to be 

 misplaced, either by an unusually warm spring or 

 autumn, which produces the mean, even though the 



^ Loudon's Gardenei's' Magazine, VoL XVII. p. 147. 

 4* 



